README.md

Is it just me, or is it crazy that directly saving a model created via #build or #new on a has_many :through relationship does not create a join model?

See the migrations.rb, models.rb & test_irb_session.txt files below for a concise example of the problem.

This kind of technique is used commonly on nested controllers, and it would work fine for a plain belongs_to or has_many associations. In fact, this behaviour would stop me from using any RESTful controller helpers, like resource_controller.

Is there are way to make it work? Or do I need to look for alternatives, like the below?

s = Subject.first
a = Article.new

# this will save the new article record and create the join model
s.articles << a

The only other place I've found mentioning is this recent bug report

current_workaround.rb

#
# This is my current workaround. See the below files for an example of the problem.
#

# This will be used when nested under a subject
class ArticlesController < ApplicationController

  # POST /subjects/1/milestones
  def create
    @subject = Subject.find(params[:subject_id])
    @article = @subject.articles.new(params[:article])

    if @article.save
      # Now assign it to the subject manually. FIXME: Ugly hack.
      @article.subjects << @subject
      @article.save!
      
      flash[:notice] = 'Created!'
      redirect_to(subjects_articles_path(@subject, @article)
    else
      render :action => 'new'
    end
  end

end

migrations.rb

class Migrations < ActiveRecord::Migration
  def self.up

    create_table :articles { |t| t.timestamps } 
    create_table :subjects { |t| t.timestamps } 

    create_table :memberships do |t|
      t.integer :article_id
      t.integer :subject_id
      t.timestamps
    end

  end
end

models.rb

class Article < ActiveRecord::Base
  has_many :memberships
  has_many :subjects, :through => :memberships
end

class Subject < ActiveRecord::Base
  has_many :memberships
  has_many :articles, :through => :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :article
  belongs_to :subject
end

test_irb_session.txt

> subject = Subject.create
  Subject Create (0.4ms)   INSERT INTO "subjects" ("created_at", "title", "updated_at") VALUES('2010-03-24 03:32:36', NULL, '2010-03-24 03:32:36')
=> #<Subject id: 1, title: nil, created_at: "2010-03-24 03:32:36", updated_at: "2010-03-24 03:32:36">

> article = subject.articles.build
=> #<Article id: nil, title: nil, created_at: nil, updated_at: nil>

> article.save
  Article Create (0.5ms)   INSERT INTO "articles" ("created_at", "title", "updated_at") VALUES('2010-03-24 03:32:49', NULL, '2010-03-24 03:32:49')
=> true

> subject.reload
  Subject Load (0.5ms)   SELECT * FROM "subjects" WHERE ("subjects"."id" = 1) 
=> #<Subject id: 1, title: nil, created_at: "2010-03-24 03:32:36", updated_at: "2010-03-24 03:32:36">

> subject.articles
  Article Load (0.3ms)   SELECT "articles".* FROM "articles" INNER JOIN "memberships" ON "articles".id = "memberships".article_id WHERE (("memberships".subject_id = 1)) 
=> []

> Membership.count
  SQL (0.3ms)   SELECT count(*) AS count_all FROM "memberships" 
=> 0